Klipsch Heresy III Loudspeaker

Efficiency Rules!

You might be wondering what is so heretical about the $1700 Heresy III, the most affordable model in Klipsch’s Heritage line. The original design was introduced circa 1957 as a center channel for a pair of Klipschorns, although the name was not used officially until 1964 in a company brochure. By 1973, Klipsch was running a national ad with “Heresy in the Church” as the by-line. It was the first non-corner speaker that Paul W. Klipsch designed, and thus it was considered heretical in the sense of violating the master’s doctrine of corner-horn loading. Ironically, the Heresy III strikes me as being even more heretical in this day and age of sub-90dB sensitivity speakers. Here you have a fairly compact package, almost bookshelf-sized, that refuses to pander to current design fashion and focuses instead on high sensitivity and maximum SPL output.

Sporting a sensitivity of 99dB it opens up a new vista of sonic exploration—namely, low-power amplification. And I mean really low-power, 5Wpc or less. In the 1950s, five watts was deemed to be a perfectly adequate power reserve for domestic applications. With the advent of low-sensitivity bookshelf speakers, power amplifier ratings started to climb, culminating in the 1980s with the new standard, a 100-to-200Wpc solid-state amplifier. The raisons d’être of high-efficiency and high-sensitivity loudspeakers is not only to accommodate the single-ended triode (SET) crowd, but also to raise the macrodynamic bar—nudging dynamic range to new heights and doing so with low distortion levels. A conventional speaker may be able to soak up 200 watts short term without damage, but it will typically starts compressing and distorting when it hits sound pressure levels in the high 90s and is in fact lucky to make it above 100dB gracefully. On the other hand, the Heresy III’s maximum output is 116dB at 1 meter. A simple point-source calculation shows that the maximum SPL at a listening distance of 4 meters (about 12 feet) will be 104dB, adequate headroom for reproducing the maximum SPL generated by a symphony orchestra, even at Row A.

The Heresy has always been a three-way closed-box design throughout its history, but it underwent two upgrades, first in 1985 when it was released as Version II and then in 2006, to coincide with the 60th anniversary of the K-Horn, when several Heritage models (Klipschorn, Heresy, LaScala, Cornwall) were updated. The responsible designer was and still is Roy Delgado, who worked directly with Paul Klipsch for many years. Version III features significant driver upgrades. First of all, a more powerful woofer was selected with smooth response (and proven reliability in Klipsch’s pro-cinema surround applications). Both the tweeter and midrange compression drivers incorporate Kilpsch’s best- effort phasing plugs to maintain coherent wave fronts, and both have been updated to titanium diaphragms for extended response and lower distortion. Roy Delgado related the following: “One of things Paul taught me was to make sure that the drivers tell you where they want to be crossed over by taking into account several parameters. Minimum or no interference in the crossover band is the goal.” The speaker ships with a slanted riser base to tilt the cabinet backward for floor placement, which is how I used and evaluated the Heresy. However, the riser is removable to allow for bookshelf (i.e., non-audiophile) installations.

I deliberately referred to the Heresy as both high-efficiency and high-sensitivity. A high speaker sensitivity does not necessarily imply high speaker efficiency. For example, it is possible to combine several low-efficiency woofers to obtain a high-sensitivity rating. Efficiency is a figure of merit for the conversion of electrical into acoustical energy. The efficiency of an 8-inch woofer radiating directly into half space is typically no better than 1%. That means that 99% of the electrical energy fed into the voice coil is dissipated as heat. No wonder heat is a voice coil’s number one enemy. Woofers do much better when radiating into tubes or pipes whereby the efficiency can increase by up to a factor of 50. However, the required mouth size for a bass horn becomes prohibitively large at low frequencies. Another approach, exemplified by the Heresy, is to increase the intrinsic efficiency of the woofer. That can be done in three ways: increasing the woofer’s cone diameter, reducing the cone mass, and increasing the magnetic-field strength. Unfortunately, there is no free lunch. Reducing the cone mass raises the resonant frequency, while overdamping the woofer with a large magnet reduces the total Q; both of these parameters combine to reduce low-frequency extension.

It should be noted that speaker sensitivity can be and often is a misleading specification. A higher sensitivity does not necessarily imply a higher maximum SPL, as that depends on the woofer’s voice-coil excursion limit and power handling. Furthermore, the reported sensitivity is greatly influenced by the measurement protocol, which may not be disclosed by the speaker manufacturer. International standards specify using a broadband test signal such as band-limited pink noise. There have been cases where a pure sinewave has been used at a frequency corresponding to a peak in the frequency response in order to pad the stated sensitivity.

Sensitivity is conventionally measured using a voltage that corresponds to one watt into the nominal impedance of the speaker. That would be 2.83V across an 8-ohm load and 2V across a 4-ohm load. The problem is that a speaker’s impedance magnitude is far from being flat with frequency, and manufacturers do not rate their speakers consistently. The box resonance of the Heresy is at 64Hz and the minimum impedance is about 4.2 ohms at 150Hz. Klipsch confirmed that it uses a 2.83V pink-noise signal over the bandwidth of the speaker to determine sensitivity and that measurements are usually performed in half space. However, according to the international standard, the nominal impedance is to be no greater than the minimum impedance times a factor of 1.25. Thus there’s an argument to be made that the Heresy should be rated as a nominal 5-ohm load instead of 8-ohm. Using a lower test signal voltage appropriate for a 5-ohm nominal load, would of course reduce the Heresy’s measured sensitivity. It turns out that Klipsch uses an integrated impedance approach to establishing the nominal impedance rating. Since the Heresy’s impedance exceeds 10 ohms above about 700Hz due to padding down of the mid and tweeter drivers, its nominal rating is more closely related to its average impedance which is at least 8 ohms. In any case, rest assured that the Heresy III had no trouble at all partnering with a 2.5Wpc SET amplifier, making it the most sensitive speaker in my house.

The midrange uses an exponential horn while the tweeter is coupled to a tractrix horn, the better choice at high frequencies due to reduced beaming. The success of any horn-loaded speaker is super-dependent on proper driver application and integration. Push the horns below their cut-off frequency and you end up with reflections from the mouth back into the horn, which result in well-known megaphone-like coloration. To its credit, the Heresy is very well integrated. The range above 700Hz is fairly smooth without any major issues. I measured a dip around 600Hz but that was a measurement artifact, while room modes were clearly evident below 300Hz. There’s plenty of midbass but essentially no deep bass. With assistance from room gain expect flat bass extension to about 55Hz.

As is typically the case with loudspeakers, a single-on axis measurement does not reveal the whole story since the soundfield is three-dimensional in nature. The problem is that the horns start beaming in the presence region, with output falling rapidly off- axis. However, listening on-axis was not an option. As a practical matter, I found it necessary to toe-in the speakers a few degrees in order to enlarge the sweet spot at the listening seat; otherwise slight head movements resulted in significant image shifts. But the downside of that was a rolled-off treble range and a tonal balance that was closed-in and dark-sounding. The somewhat “heretical” antidote was to ignore the floor riser and actually raise the speakers off the floor by about two feet so as to position the tweeter at about ear level. The end result was a much more neutral balance. Stand-mounting the Heresy, as one would do with monitor-style speakers, offered another important benefit. Eliminating the floor loading reduced the magnitude of the midbass response hump, which was responsible for plummy, muddled bass lines that even a break-in period is also mandatory. Right out of the box the mids sounded distinctly veiled. But not to worry—the fog lifted after several hours of play time to reveal a lucid and cogent midrange.

The Heresy was extremely revealing of a power amp’s sonic character, especially the quality of the first watt. It’s the first watt that sets the stage for a high-efficiency speaker. High- power Class AB amps that sound perfectly fine in the context of ordinary-sensitivity speakers didn’t perform nearly as well as low-power SET amplifiers, lending support to the notion that the next 99 watts hardly matter if that first watt is compromised. In particular, two SET amplifiers stood out: the 3Wpc Get*Set*Go designed by DIYTube’s Shannon Parks and based on the 6B4G triode, and Pete Millett’s R120 SET, which, as the name implies, is based on the French R120 power triode. The Get*Set*Go yielded a cohesive and transparent soundstage populated by well-focused image outlines. In particular, it catered to female voice with smooth, extended upper registers and nary a hint of brightness. Bass lines were decent, but hampered by an excess of midbass energy when the Heresy was sitting on the floor.

As good as the Get*Set*Go was, the R120 elevated the Heresy to an even higher performance plateau. Textures were sweeter sounding, tonal colors more vivid, and image outlines even more palpable. Treble detail and resolution of ambient info were improved. Bass lines tightened up to a degree that no pentode or ultra-linear push-pull amp could match. And above all else, I was swept away by the emotional intensity unleashed by this combo. It was hard to believe that only 2.5Wpc were in play. Higher-power amps can, of course, play louder, but the goosebump factor with this SET when amplifying a singer’s whisper or plaintive cry was almost off the chart. And while I could drive the R120 into clipping, albeit soft-clipping, on a 96dB load, it seemed to possess adequate headroom driving the Heresy—at least in a medium-sized listening room with sane musical selections.

I don’t mean to imply that because the Heresy proved to be a sonic revelation with low-power SET amplifiers it might be unhappy with more power. It was quite comfortable being partnered by the 120W Z-Infinity Z120 monoblocks, and in fact, such power reserve is a prerequisite for testing its ultimate SPL limit. It was also happy being partnered by the Lamm Audio M1.2 Reference monoblocks, which yielded smooth textures and spectacular dynamic reach.

The Heresy can play louder and with lower distortion than a host of rear-horn-loaded full-range drivers, many of which are handicapped by a short voice coil and thus poor excursion capability. It is a godsend for music lovers in search of a tube- friendly high-sensitivity speaker. The Heresy is not perfect, but then no one should expect perfection at its asking price. The real surprise is that it is capable of delivering a significant slice of sonic heaven. It represents a terrific value and merits nothing less than an enthusiastic two-thumbs-up recommendation!